1. Technical Field
The present invention relates to a film forming method, a method for manufacturing an organic electroluminescent device, an organic electroluminescent device, and an electronic apparatus.
2. Related Art
When forming a film such as a light-emitting layer of, for example, an organic electroluminescent device (hereinafter also referred to as an organic EL device), there is used a technique in which on a substrate ink, that is obtained by dissolving in a solvent composed of a plurality of species of liquid mixed with each other a functional material that forms a light-emitting layer, is applied and then dried, thereby forming a film.
There are mainly two functions which are demanded of the ink. One function is capability to be applied on the substrate, and the other is ability to form a flat film. It is very difficult for solvent with a single component to fulfill those two functions. Therefore, there is often used, for example, ink composed of a plurality of species of solvent mixed with each other (see, for example, JP-A-2004-535653). Further, when drying the ink, the pressure surrounding the ink is reduced from a normal atmosphere pressure to 3-10 Pa at a constant speed, thereby allowing the solvent to evaporate (see, for example, JP-A-2004-127897).
As shown in FIG. 15, when drying the ink applied, the evaporation rate of the solvent is higher in a central part of the ink 150 than in the peripheral part thereof. Note that FIGS. 15 and 16 each show a view illustrating a step for forming a hole injection layer of an organic EL device. Reference symbols 131, 135 and 136 respectively denote an anode of the organic EL device, a pixel aperture film, and a partition. The flow of the solvent from the central part flows towards the peripheral part of the ink 150 so as to replace the reduced solvent which has evaporated (the flow is indicated by solid arrows in the ink). It is known that the flow rate at this time is proportionate to the difference between the evaporation amount of the central part and the evaporation amount of the peripheral part. Due to this flow of the solvent, solute in the ink flows from the central part towards the peripheral part and remains there. Accordingly, there occurs a concentration gradient between the central part and the peripheral part of the ink. On the other hand, the solute tends to diffuse in the entire ink so as to eliminate the concentration gradient (this tendency is indicated by open arrows in the ink). In this way, in the ink, there is observed an equilibrium state between the solute moving in accordance with the flow of the solvent and the solute tending to diffuse.
JP-A-2004-535653 and JP-A-2004-127897 are examples of related art.
As the evaporation takes place more vigorously upon dying under reduced pressure, however, the difference in the evaporation amount between the central part and the peripheral part of the ink becomes larger. Accordingly, the flow of the solvent becomes dominant over the diffusion of the solute, thereby making it impossible for the solute to diffuse in the entire ink. As a result, the solute stacks in the peripheral part, so there arises a problem in that after evaporation of the solute, only such a film (for example, the hole injection layer) 132 can be obtained that the central part is thinner and the peripheral part is thicker, as shown in FIG. 16. On the other hand, when the drying speed is reduced, it takes much time to perform drying, which disadvantageously reduces the throughput.